Video cameras employing infrared (IR) sensitive imaging elements have heretofore relied on cryogenically cooled IR sensitive focal plane arrays (FPA) or sensors with an associated cryogenic subsystem for maintaining the FPA sensor at temperatures in the range of 60.degree. K. to 80.degree. K. By stabilizing the temperature of the FPA sensor at temperatures which provide substantially zero thermal or infrared radiation emmission, any pixel to pixel temperature dependent response variations of individual sensor pixel elements is substantially eliminated since the sensor signal in response to thermal energy generated at cryogenic temperatures is substantially zero. Furthermore, by quickly removing thermal energy absorbed by or conducted to the sensor with the cryogenic subsystem, the signal generated by the sensor can be substantially attributed to the infrared radiation reaching the sensor from a scene to be imaged. Thus a very high signal to noise ratio is maintained.
It is also known to add a hollow cylindrical cold shield over the FPA sensor active surface to define its field of view. Such a cold shield provides a limiting aperture which defines a solid angle field of view of the active surface of the FPA sensor. By also maintained the cold shield at cryogenic temperatures a uniform temperature object having substantially zero infrared radiation emmission is provided at the edges of the field of view of each pixel of the FPA sensor active surface. It is also known to provide a cold shield having internal surfaces with high emmissivity for absorbing any stray infrared radiation not incident on the active surface and to provide external surfaces with low emmissivity at infrared wavelengths to reflect any infrared radiation away from the cold shield. Such a cold radiation shield and a method for making is disclosed e.g. by Du Pree et al. in U.S. Pat. No. 5,277,782.
By maintaining the FPA sensor active surface and the cold shield at cryogenic temperatures, a uniform substantially zero DC background signal is provided by each pixel of the FPA sensor active area in the absence of an infrared scene, i.e. when a camera aperture is blocked by a shutter. Cryogenically cooled systems provide good sensitivity to IR radiation at low levels and a high contrast IR image scene signal by providing a high signal to noise ratio. Cryogenically cooled IR imaging systems have been used for military applications, e.g. in missile tracking or night vision systems, but the high cost and difficulties associated with maintaining a cryogenic subsystem have limited commercial use.
More recently, microbolometer detector arrays or uncooled IR focal plane arrays (UFPA) operating near room temperature have allowed the elimination of cryogenic subsystems thereby reducing cost and complexity in infrared video cameras systems. An example of an IR video camera employing a UFPA is given by Wood in U.S. Pat. No. 5,420,419. Wood provides a microbolometer focal plane array housed within a vacuum chamber, which is sealed by a window invisible to the infrared wavelengths. A radiation receiving system provides a lens to image an infrared scene onto the active surface of the FPA and an iris for defining the solid cone angle of a field of view. A thermoelectric cooler, (TEC), attached to the back of the focal plane array replaces the cryogenic cooling apparatus of earlier systems. A closed loop temperature controller and temperature sensor, mounted proximate to the UFPA, stabilize the temperature of the focal plain array and associated housing at a substantially constant temperature of 22.degree. C.
Such a system offers the benefit of lower cost, however, the microbolometer detector array suffers from excessive pixel to pixel non-uniformity's, due to its construction, as well as the drawback that since the detector array is maintained at room temperature, a DC background signal or dark signal, provided by each pixel in the absence of an IR scene, is substantially higher in an uncooled system than in a cryogenically cooled system and in some applications, the DC background signal amplitude can be much greater than the signal amplitude of the infrared scene to be imaged. Thus, the cost benefits gained by eliminating a cryogenic subsystems have been partially offset by the need to add a complex image signal processing apparatus to the camera electronics modules.
Examples of video electronic signal processing systems are given in U.S. Pat. No. 5,489,776, issued to Lung for a microbolometer FPA, and U.S. Pat. No. 5,528,035, issued to Masarik, for a pyroelectric FPA, both assigned to Hughes Aircraft Company. These examples address the problem of eliminating excessive pixel to pixel non-uniformity's in UFPA's and remove DC offset biases including the detector DC background signal and DC offset signals generated by system electronic components.
To calibrate a signal processing apparatus as described above, scene radiation is blocked, e.g. by an iris as described by Wood, cited above, and a dark signal is generated by each pixel of the FPA while it is shielded by the iris. The system dark response, given by the average dark signal response of each pixel, is stored on a pixel by pixel basis in a memory contained within a camera electronics module. When an infrared scene is imaged, the stored dark signal response is processed in combination the signal produced when the camera is illuminated by an IR scene and the scene signal alone is extracted and further amplified, thereby providing a corrected video scene image signal.
In radiometer applications where the absolute temperature of a scene is desired to be measured, a further step may include determining a response of the camera system while illuminated by a calibrated black body radiator radiating at a known black body temperature. The black body response of the system is then stored in a memory for later comparison to an infrared scene to be measured. This final step is beneficially performed in a factory environment where control of the process can be more readily maintained.
Numerous applications of IR video cameras require the use of various imaging lens of variable focal length to optimize resolution. In order to maximize the use of each pixel of the detector array, the IR scene to be imaged should fill the entire array area. For this reason, cameras are made available with variable focal length as well as interchangeable imaging optical systems having a wide range focal lengths and fields of view. Applications range from very long focal lengths in excess of 200 mm to microscope objectives having focal lengths in the range of 0.10 mm. By interchanging imaging optical systems the average dark signal response of at least some pixels in the array, especially those pixels near the edges of the array, can vary as the system field of view and the configuration of the imaging optical system varies. It has therefore been necessary to recalibrate the average system dark signal response each time a new imaging optical system is mounted on the camera. Furthermore, in radiometer applications where the black body temperature of an infrared scene image is determined by comparing the camera response for the infrared scene to be imaged with the camera response for a black body radiator of known black body temperature, a factory recalibration has been required for each imaging optical system which is intended to be used with the camera system.
Thus, the camera system dark signal response is periodically determined in the field by providing a shutter for blocking the camera aperture. Such a procedure is normally performed, e.g. each time the camera is powered up such that a new camera system dark signal response is stored in memory for each use. It is also typical that a camera system be recalibrated i.e. determining the average dark signal response each time the imaging optics of the camera system are interchanged with another imaging optics assembly.
In radiometric measurement applications, the calibration of the system response of a camera to a black body radiator of known black body temperature is performed in the factory. The calibration is performed with camera system maintained at a known ambient temperature and with a particular imaging optics assembly installed. A system black body response is stored in a memory module for use in the field. In the field when a different imaging optics assembly is installed, the stored system black body response no longer directly applies and a error in the measured temperature of the infrared scene to be imaged can occur unless a new system response is reestablished in a factory setting. This is an inconvenient to the user.
An additional disadvantage of present systems is that as the camera system and associated optical and electronic components are frequently cycled through various ambient temperature environments, as well as through a camera system electronic power-up cycle. These varying conditions cause the temperature of the camera system to slowly drift in time. Thus the camera system itself can emit varying amounts of infrared radiation which may drift across the field of view of the detector array causing the instantaneous system dark signal response to vary from a dark signal response stored in a memory module at the most recent dark signal calibration. This slowly drifting instantaneous dark signal response ultimately degrades imaging performance.
Thus, a need exists in the art to provide a low cost video camera with the ability to operate over a range of ambient temperature conditions while still maintaining a constant imaging performance.
A further need exists in the art to provide a low cost video camera which can adapt to perform with a variety of interchangeable imaging optical systems for a variety of video imaging applications while still maintaining a constant imaging performance.
A still further need exits in the art for a low cost video radiometer which can adapt to perform radiometric measurements over a range of ambient temperature conditions and with a variety of interchangeable imaging optical systems for a variety of applications while still maintaining accurate temperature measurement performance without having to be recalibrated in a factory environment.
Accordingly it is a primary object of the present invention to provide a low cost infrared video camera with a substantially uniform dark signal response over a wide range of operating temperatures.
It is a further object of the present invention to provide a low cost infrared video camera which can adapt to perform with a variety of interchangeable imaging optical systems while still maintaining a substantially uniform dark signal response from one interchangeable optical system to another.
It is a still further object of the present invention to provide a low cost video radiometer which can adapt to perform radiometric measurements over a range of operating temperature conditions while providing an accurate absolute temperature measurements.
It is another object of the present invention to provide a low cost video radiometer which can adapt to perform radiometric measurements with a variety of interchangeable imaging optical systems for a variety of applications while providing an accurate absolute temperature measurement without having to recalibrate the black body response of the radiometer system for each interchangeable imaging optical system in a factory environment.